This invention is in the field of transistorized amplifiers, and relates more specifically to linear differential transconductance amplifiers.
The basic concept of using a differential correction amplifier to provide a correction signal to a main differential amplifier is shown in U.S. Pat. No. 4,146,844. Such an amplifier circuit as disclosed in that patent is shown in FIG. 1. This basic cascomp amplifier circuit has a correction amplifier consisting of transistors 14 and 16, resistor 18 and current sources 32 and 34, which takes its input from the collectors of the two input transistors 22 and 26 of the main amplifier. This circuit provides a first order nonlinearity correction of the main amplifier. However, the corrected gain will be dependent on transistor beta since the voltage presented to the correction amplifier is not the exact error voltage of the main amplifier. Further, an additional cascode output stage is required to reduce the Miller capacitance of the correction amplifier. Without this additional stage, undesirable ringing in the output current signal would occur. Lastly, the reverse breakdown voltage of the input stage, as in all simple differential pairs, is limited to the reverse breakdown voltage of one emitter base junction.
These limitations become apparent upon a closer analysis of the prior art cascomp amplifier shown in FIG. 1. In this circuit, transistors 22 and 26 form the differential input pair, the emitters of which are coupled through emitter resistor 24. The differential voltage (+VIN, -VIN) is applied to the bases of transistors 22 and 26, and the source resistance of the differential voltage input is shown by resistors 20 and 28. Current sources 30 and 36 provide the bias current to the input transistor pair. Transistors 10 and 12 form a common base amplifier stage which, in conjunction with transistors 22 and 26, form a cascode output stage. The voltage difference across the emitters of transistors 10 and 12 provides an approximation of the error voltage found in the base emitter voltage of the input pair of transistors 22 and 26. The error voltage is presented to a correction amplifier. The correction amplifier includes a differential pair of transistors 14 and 16, with their emitters coupled through emitter resistor 18. Current sources 32 and 34 provide the bias current to the transistors 14 and 16. The collector current of these two transistors is proportional to the error voltage found in the differential input pair. Accordingly, the error current generated by transistors 14 and 16 is added to the differential collector current of transistors 10 and 12, and a linear current is produced.
Although the prior art cascomp amplifier provides a linear output current, the gain of the amplifier is a function of transistor beta. The emitter currents of transistors 10 and 12 are the emitter currents of transistors 22 and 26, reduced by their respective transistor alphas. Therefore, the error voltage across the emitters of transistors 10 and 12 is not an exact copy of the error voltage found in the input transistors 22 and 26, but is slightly reduced, and by an amount which is dependant upon transistor beta. The gain of the cascomp amplifier is made even more sensitive to transistor beta by the alpha loss of the cascode output stage which includes transistors 10 and 12.
The prior art cascomp amplifier has two additional limitations. If a resistive load is connected to the collectors of transistors 10 and 12, the large reflected capacitance of the correction amplifier will be presented to the collectors of the input pair, transistors 22 and 26. This, in combination with the inductive load presented by the emitters of transistors 10 and 12, creates a resonant tank circuit at the collectors of transistors 22 and 26. In turn, this resonant circuit produces unwanted ringing in the transient response of the amplifier. This ringing may be reduced by connecting an additional common base stage to the collectors of transistors 10 and 12, but this will be at the expense of an increased sensitivity to transistor beta. Finally, the reverse breakdown voltage of the input pair is limited to the reverse breakdown voltage of one of the input transistors.
What is desired is a cascode amplifier which provides an improvement in the beta dependency of the gain, eliminates excessive ringing of the transient response without an additional cascode output stage and also provides an improvement in the reverse breakdown voltage of the input stage while maintaining linear operation over a wide dynamic range.